feat(rules): add NodeFileDescriptorLimit kernel exhaustion alert

Add a new alert when fs.file-nr is close to fs.file-max

Signed-off-by: Loic Blot <loic.blot@unix-experience.fr>

docs/node-mixin/alerts/alerts.libsonnet

@@ -277,6 +277,38 @@
               description: "At least one device in RAID array on {{ $labels.instance }} failed. Array '{{ $labels.device }}' needs attention and possibly a disk swap.",
             },
           },
+          {
+            alert: 'NodeFileDescriptorLimit',
+            expr: |||
+              (
+                node_filefd_allocated{job="node-exporter"} * 100 / node_filefd_maximum{job="node-exporter"} > 70
+              )
+            ||| % $._config,
+            'for': '15m',
+            labels: {
+              severity: 'warning',
+            },
+            annotations: {
+              summary: 'Kernel is predicted to exhaust file descriptors limit soon.',
+              description: 'File descriptors limit at {{ $labels.instance }} is currently at {{ printf "%.2f" $value }}%.',
+            },
+          },
+          {
+            alert: 'NodeFileDescriptorLimit',
+            expr: |||
+              (
+                node_filefd_allocated{job="node-exporter"} * 100 / node_filefd_maximum{job="node-exporter"} > 90
+              )
+            ||| % $._config,
+            'for': '15m',
+            labels: {
+              severity: 'critical',
+            },
+            annotations: {
+              summary: 'Kernel is predicted to exhaust file descriptors limit soon.',
+              description: 'File descriptors limit at {{ $labels.instance }} is currently at {{ printf "%.2f" $value }}%.',
+            },
+          },
         ],
       },
     ],